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Re: A-4 Repair [message #26553 is a reply to message #26502]

Sun, 11 February 2018 15:41

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

First off, there are no Germanium transistors in these amps. Only the very early Lot-O-Tone heads had them and they failed so often that they were dumped for Silicon transistors. NOS transistors are great if you are working in a museum and want to keep things absolutely original. I use modern replacements that are cheaper, quieter and often higher rated that the originals. Your amp sounds like it has already been worked over a few times, so I wouldn't worry about originality.

If I fully understand what you describe as the fuzzy distortion, it sounds like a perfect description of crossover distortion in the power amp. At least that is what I would look at as a starting point.

Crossover distortion occurs when the power amp circuit is under biased and the two halves of the audio signal don't transition smoothly from positive to negative polarities. Instead of transitioning perfectly in the middle, the first half shuts off before the second half turns on, creating a flat spot between the two signal halves.

Check the voltages at each end of the string of three bias diodes. There should be somewhere between 1.5 volts to 2 volts difference between the two ends of the diode string. This is a very delicate part of the power amp circuit because of the temperature sensing diode that is thermally connected to the heat sink. Be careful if you move this diode as the leads have been known to snap off at the case and the ends can also come unsoldered from the pc board. If the diode string is broken and the amp is on or turned on, the two halves of the power amp both turn on at the same time, causing the driver and power transistors to go up with a puff of smoke.

You can add heat shrink tubing to the metal tips of your meter leads leaving only a small tip exposed to avoid the tip from shorting things as you probe. I have a set of spring loaded clip adapters that I can use when things are really tight. With these you really have to attach the lead with the circuit power off and then turn it on and get your reading.

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[Message index]

		A-4 Repair By: Jonomega3 on Sat, 27 January 2018 08:20
		Re: A-4 Repair By: chicagobill on Sat, 27 January 2018 12:26
		Re: A-4 Repair By: Jonomega3 on Sun, 28 January 2018 00:28
		Re: A-4 Repair By: chicagobill on Sun, 28 January 2018 14:13
		Re: A-4 Repair By: Jonomega3 on Sun, 28 January 2018 15:44
		Re: A-4 Repair By: stevem on Mon, 29 January 2018 08:18
		Re: A-4 Repair By: Jonomega3 on Mon, 29 January 2018 21:16
		Re: A-4 Repair By: chicagobill on Mon, 29 January 2018 23:48
		Re: A-4 Repair By: stevem on Tue, 30 January 2018 12:07
		Re: A-4 Repair By: Jonomega3 on Tue, 30 January 2018 13:24
		Re: A-4 Repair By: stevem on Tue, 30 January 2018 15:24
		Re: A-4 Repair By: Jonomega3 on Wed, 31 January 2018 12:47
		Re: A-4 Repair By: stevem on Thu, 01 February 2018 08:23
		Re: A-4 Repair By: Jonomega3 on Thu, 01 February 2018 11:13
		Re: A-4 Repair By: Jonomega3 on Thu, 08 February 2018 15:12
		Re: A-4 Repair By: stevem on Fri, 09 February 2018 08:32
		Re: A-4 Repair By: Jonomega3 on Fri, 09 February 2018 12:54
		Re: A-4 Repair By: chicagobill on Fri, 09 February 2018 15:39
		Re: A-4 Repair By: Jonomega3 on Fri, 09 February 2018 17:07
		Re: A-4 Repair By: Jonomega3 on Fri, 09 February 2018 18:30
		Re: A-4 Repair By: chicagobill on Fri, 09 February 2018 23:46
		Re: A-4 Repair By: Jonomega3 on Sat, 10 February 2018 12:00
		Re: A-4 Repair By: chicagobill on Sat, 10 February 2018 23:54
		Re: A-4 Repair By: Jonomega3 on Sun, 11 February 2018 00:27
		Re: A-4 Repair By: stevem on Sun, 11 February 2018 06:24
		Re: A-4 Repair By: Jonomega3 on Sun, 11 February 2018 10:58
		Re: A-4 Repair By: chicagobill on Sun, 11 February 2018 15:41
		Re: A-4 Repair By: Jonomega3 on Sun, 11 February 2018 21:05
		Re: A-4 Repair By: Jonomega3 on Mon, 12 February 2018 10:10
		Re: A-4 Repair By: stevem on Mon, 12 February 2018 07:29
		Re: A-4 Repair By: Jonomega3 on Mon, 12 February 2018 09:04
		Re: A-4 Repair By: stevem on Mon, 12 February 2018 10:18
		Re: A-4 Repair By: Jonomega3 on Mon, 12 February 2018 10:41
		Re: A-4 Repair By: chicagobill on Mon, 12 February 2018 12:47
		Re: A-4 Repair By: Jonomega3 on Mon, 12 February 2018 13:25
		Re: A-4 Repair By: chicagobill on Mon, 12 February 2018 14:25
		Re: A-4 Repair By: Jonomega3 on Mon, 12 February 2018 16:07
		Re: A-4 Repair By: chicagobill on Mon, 12 February 2018 18:08
		Re: A-4 Repair By: stevem on Tue, 13 February 2018 07:39
		Re: A-4 Repair By: Jonomega3 on Tue, 13 February 2018 11:04
		Re: A-4 Repair By: chicagobill on Tue, 13 February 2018 16:40
		Re: A-4 Repair By: Jonomega3 on Thu, 26 July 2018 12:43
		Re: A-4 Repair By: chicagobill on Fri, 27 July 2018 00:02
		Re: A-4 Repair By: Jonomega3 on Sat, 28 July 2018 14:31
		Re: A-4 Repair By: stevem on Sun, 29 July 2018 06:06
		Re: A-4 Repair By: Jonomega3 on Sun, 29 July 2018 14:10
		Re: A-4 Repair By: Jonomega3 on Thu, 16 August 2018 11:45
		Re: A-4 Repair By: stevem on Thu, 16 August 2018 12:40
		Re: A-4 Repair By: Jonomega3 on Thu, 16 August 2018 13:14
		Re: A-4 Repair By: Jonomega3 on Thu, 16 August 2018 18:51
		Re: A-4 Repair By: stevem on Fri, 17 August 2018 06:51
		Re: A-4 Repair By: Jonomega3 on Fri, 17 August 2018 06:58
		Re: A-4 Repair By: stevem on Sun, 19 August 2018 06:15
		Re: A-4 Repair By: Jonomega3 on Mon, 20 August 2018 12:18
		Re: A-4 Repair By: stevem on Mon, 20 August 2018 12:27
		Re: A-4 Repair By: Jonomega3 on Fri, 24 August 2018 18:22
		Re: A-4 Repair By: chicagobill on Sat, 25 August 2018 00:15
		Re: A-4 Repair By: Jonomega3 on Sat, 25 August 2018 07:58
		Re: A-4 Repair By: Jonomega3 on Sat, 25 August 2018 11:43
		Re: A-4 Repair By: chicagobill on Sat, 25 August 2018 12:41

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